AC voltage meter is a very commonly used device in daily life. Traditionally, AC voltage meter uses a resistor divider with large resistance and includes several capacitors. Naturally, the AC voltage meter becomes a low-pass filter. For an AC source with frequency higher than 500 Hz, it is hard to perform precise measurement due to the decrease of signal strength.
Please refer to FIG. 1A and FIG. 1B. FIG. 1A shows a traditional design of an AC voltage meter which includes two resistors R1 and R2 for use as a resistor divider, and a capacitor C1 to form a detecting unit to obtain the AC voltage for further measurement. FIG. 1B shows how the signal strength changes along with frequency of an AC voltage source. It is obvious that the signal strength drops after 1 kHz. When the frequency is over 10 kHz, the AC voltage meter is hard to use for AC voltage measurement.
For some higher end AC voltage meters, the detecting unit would be finely designed to mitigate the situation mentioned above. Please refer to FIG. 2A and FIG. 2B. Two capacitors C2 and Cs are used, wherein the capacitor Cs is a variable capacitor. Thus, such design can be able to measure AC voltage sources with wider frequencies.
Conventionally, a method of compensating frequency to achieve the variable capacitor corresponding to different frequencies of AC voltage sources is to change external capacitors of the AC voltage meter manually. However, when capacitors are changed by welding, there will be excess solder residue. The capacitance values could easily lead to instability due to reasons such as temperature and humidity. Capacitance of the external capacitors will also be affected by different batches of PCB with different correction values. If the AC voltage meter needs a final tuning by production labors after it is assembled, its cost increases and precision drops.
European Patent Publication No. EP 2,386,868 thought of to overcome such problem by applying a known voltage signal across a first and second inputs to a capacitive divider circuit; measuring a voltage across a first and second plates of a third capacitor; determining a transfer function that would be operative to change the measured voltage; configuring the compensator circuit with the determined transfer function; and measuring a high voltage AC signal with the voltage measurement circuit, such that the compensator circuit is operative to change a voltage received at its input in accordance with the determined transfer function and to provide the changed voltage at its output.
Although the transfer function and compensator circuit can be used to obtain a more precise measuring result of a high AC voltage, it can not be applied to smaller range of voltages.
Therefore, an AC voltage meter which is able to measure voltages in a large range of frequencies is desperately desired.